dnd: add a style selector for the dnd style class
dnd: add a style selector for the dnd style class

It's a stock GTK class and it's used by Nautilus, so it should be
supported here.

ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,Distrotech/gnome-themes-standard,ChrisCummins/gnome-themes-standard,Distrotech/gnome-themes-standard,ChrisCummins/gnome-themes-standard,ChrisCummins/gnome-themes-standard,magcius/gnome-themes-standard,cosimoc/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,rgcjonas/adwaita-semidark,ChrisCummins/gnome-themes-standard,magcius/gnome-themes-standard,rgcjonas/adwaita-semidark,cosimoc/gnome-themes-standard,cosimoc/gnome-themes-standard,cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark,DoctorJellyface/gnome-themes-standard,magcius/gnome-themes-standard